Methane is available in large quantities in many areas of the world. Some methane is generated from refinery applications while large amounts of methane, as the principal constituent of natural gas, are found in deposits in various areas.
Methane can be used directly as a gas for heating purposes and the like when the source of methane is relatively close to the end user. However, if the methane must be transported over long distances, the methane is preferably transported as a liquid.
Methane is also used as a starting material for the production of hydrocarbons. The conversion of methane is normally carried out in a two-step procedure involving reforming the methane to produce hydrogen and carbon monoxide, synthesis gas (syngas), and then converting the syngas to higher hydrocarbons in a Fischer-Tropsch type reaction. Both steps of the process are well-known and can be readily illustrated: the first step by U.S. Pat. Nos. 1,711,036, 1,960,912 and 3,138,438; the second step by U.S. Pat. Nos. 4,477,595, 4,542,122 and 4,088,671.
The present invention is primarily concerned with the second step, the well-known hydrocarbon synthesis or Fischer-Tropsch reaction. To this end, the present invention provides an improved catalyst for selectively converting syngas to hydrocarbons.
Many attempts at providing effective catalysts for selectively converting syngas to hydrocarbons have previously been disclosed in the art.
Chester et al., U.S. Pat. No. 4,523,047 disclose employing a catalyst system comprising zeolite ZSM-45 in combination with tungsten, vanadium, molybdenum, rhodium, nickel, cobalt, chromium, manganese, platinum or lead to produce liquid hydrocarbons from syngas in a Fischer-Tropsch synthesis slurry reactor system.
Cobalt containing catalysts are also well-known in the art for use in Fischer-Tropsch synthesis. Payne et al., U.S. Pat. No. 4,542,122 disclose a catalyst composition comprising cobalt or thoria promoted cobalt on a titania support for converting syngas to C.sub.10+ linear paraffins and olefins. Further, a number of prior art disclosures describe employing various iron-cobalt spinels as catalysts in Fischer-Tropsch synthesis. See, for example, Soled et al., U.S. Pat. No. 4,518,707 (high surface area iron-cobalt spinels which are fully reduced/carburized to selectively convert syngas to alpha-olefins); Fiato et al., U.S. Pat. No. 4,537,867 (promoted iron-cobalt spinels containing low levels of cobalt to selectively convert syngas to C.sub.2 to C.sub.6 olefins with low CH.sub.4 production); Soled et al., U.S. Pat. No. 4,544,671 (high surface area iron-cobalt spinels which are fully reduced/carburized to selectively convert syngas to alpha-olefins); Fiato et al., U.S. Pat. No. 4,544,672 (reduced and carbided unsupported iron-cobalt single phase spinels containing low levels of cobalt to selectively produce low molecular weight olefins); Fiato et al., U.S. Pat. No. 4,544,674 (alkali promoted iron-cobalt single phase spinels containing low levels of cobalt to selectively produce low molecular weight olefins); Soled et al., U.S. Pat. No. 4,584,323 (copper promoted iron-cobalt spinels to convert syngas to alpha olefins); and Soled et al., U.S. Pat. No. 4,607,020 (copper promoted iron-cobalt spinels carbided in-situ in the reactor to selectively convert syngas to alpha-olefins).
Also known in the art for use in Fischer-Tropsch hydrocarbon synthesis are ruthenium based catalysts. For example, Madon, U.S. Pat. No. 4,477,595, describes employing ruthenium catalysts supported by titanium oxide, niobium oxide, vanadium oxide or tantalum oxide to produce C.sub.5 to C.sub.40 hydrocarbons in a Fischer-Tropsch hydrocarbon synthesis process; and Wachs et al., U.S. Pat. No. 4,861,747 describe a catalyst comprising ruthenium supported on a non-crystalline surface-modifying oxide containing titania support for producing substantially alcohol free hydrocarbon products having high concentrations of internal olefins in a Fischer-Tropsch hydrocarbon synthesis process. Further, cobalt-ruthenium catalysts for Fischer-Tropsch hydrocarbon synthesis have been described in, e.g., Iglesia et al., U.S. Pat. No. 4,738,949 and Iglesia et al., U.S. Pat. No. 4,822,824 (cobalt and ruthenium deposited on a titania support).
In addition, copper promoted iron-manganese catalysts are described in Fiato et al., U.S. Pat. No. 4,618,597 for the conversion of CO/H.sub.2 into alpha olefins. The iron-manganese spinels are prepared by utilizing an alpha-hydroxy aliphatic carboxylic acid which acts as a solubilizing agent for the iron and manganese salts in aqueous solution. Representative examples of such acids are given as glycolic, malic, glyceric, mandelic, tartaric, lactic acids and mixtures thereof.
Also of interest is Kim et al., U.S. Pat. No. 4,624,968 which discloses a two stage Fischer-Tropsch hydrocarbon synthesis process wherein an iron-based catalyst, e.g., iron/cesium/zinc/potassium, iron/manganese/potassium, iron/cobalt/potassium, is employed in the first stage to selectively produce olefins; and a ruthenium-based catalyst, e.g., ruthenium/titanium oxide, ruthenium/aluminum oxide, ruthenium/niobium oxide, ruthenium/silicon oxide, is employed in the second stage to selectively produce paraffins.
Cobalt-oxide spinel catalysts have also been described in the literature. Fornisari et al., "Cobalt Mixed Spinels as Catalysts for the Synthesis of Hydrocarbons," Ind. Eng. Chem. Res., Vol. 26, No. 8, pp. 1500-1505 (1987), reports that catalysts consisting of cobalt, copper, zinc, and chromium mixed oxides have improved selectivity to hydrocarbons where the catalyst contain comparable amounts of cobalt and copper. Selyama et al., "Characterization and Activity of Some Mixed Metal Oxide Catalysts," Ind. Eng. Chem. Prod. Res. Dev., Vol. 24, No. 1, pp. 19-27 (1987), reports that spinel type oxides, e.g. CuCo.sub.2 O.sub.4 and CoNiO.sub.4, show activity and selectivity for biacetyl formation. Van der Riet et al., "Selective Formation of C.sub.3 Hydrocarbons from CO+H.sub.2 using Cobalt-Manganese Oxide Catalysts," J. Chem. Soc., Chem. Commun., pp. 798-799 (1986) reports that selective formation of C.sub.3 hydrocarbons is obtained when cobalt-manganese oxide catalysts are employed in hydrocarbon synthesis processes.
However, despite the prior art disclosures there still exists a need in the art for a catalyst which under slurry Fischer-Tropsch conditions selectively produces hydrocarbons, especially olefins and higher paraffins, with a high conversion of carbon monoxide and low methane production.